1. Field of the Invention
This invention relates to detection of an EMG signal and, more particularly, to a method and apparatus that produces a model diaphragm EMG signal, which can be utilized, for example, to monitor the condition of a patient and/or synchronize the operation of a ventilator to the breathing cycle of a patient.
2. Description of the Related Art
Ventilators used to promote the exchange of air in the lungs of a patient are well-known in the art. Ventilators operate by urging air into the lungs of the patient during inhalation and by terminating urging air into the patient""s lungs during exhalation. In a normal patient, the inhalation and exhalation of air into and out of the lungs are accomplished by activation and relaxation of the patient""s respiratory muscles, and, in particular, the diaphragm muscles, which contract and relax in response to a signal from the phrenic nerve. The activation of the diaphragm produces an electromyographic (EMG) signal and, more particularly, a diaphragm EMG signal, that can be measured. This diaphragm EMG signal is generally representative of the respiratory effort generated by the patient during each breath cycle.
The diaphragm EMG signal can be used for a variety of purposes, from monitoring the respiratory function of the patient to controlling a ventilator that assists the patient in breathing. For example, in general, some conventional ventilators operate on the principle that each inhalation by a patient has the same interval. Accordingly, if the interval of the patient""s diaphragm EMG signal during inhalation is longer or shorter than the inhalation interval of the ventilator, the ventilator will provide to the patient more or less air, respectively, than the patient desires, with corresponding patient discomfort.
Conventional ventilators have attempted to utilize a measured EMG signal, and, in particular, the measured diaphragm EMG signal to control the supply of air or other breathing gas to the lungs of the patient. However, because the measured EMG signal contains the patient""s diaphragm EMG signal, an electrocardiogram (ECG) signal, and other noise, such as noise due to movement between sensing electrodes and tissue of the patient during breathing, difficulties are encountered in synchronizing the operation of the ventilator with the diaphragm EMG signal of the patient.
A variety of techniques has been utilized to suppress or eliminate the contribution of an ECG signal and noise from the measured EMG signal to obtain a model of the EMG signal, i.e., a xe2x80x9ccleanxe2x80x9d EMG signal, such as a clean diaphragm EMG signal, which corresponds to the EMG signal that is produced directly by the diaphragm. One conventional technique for producing a clean diaphragm EMG signal includes clipping the top of the QRS complex of the measured EMG signal. However, this technique is unsatisfactory because it leaves the majority of the QRS complex and may introduce new artifact harmonics to the frequency spectrum. Another technique includes replacing, for the duration of each QRS complex of each ECG cycle, the measured EMG signal with the value of the measured EMG signal recorded immediately prior to that QRS complex. A problem with this technique is that it leaves the remainder of the ECG cycle, which includes most of the low frequency power. In another conventional technique, computerized processing is utilized to subtract an ECG signal obtained during relaxation from the measured EMG signal. A problem with this technique is that the ECG signal will vary with effort, due to changes in both heart rate and recording conditions, which introduces artifacts. In yet another technique, the EMG signal is sampled between one T wave and a subsequent QRS complex. Such recordings have been utilized in spectral analysis of human diaphragm EMG signals. A problem with this technique is that the measured EMG signal is not sampled between the Q wave and the T wave of each ECG cycle thereby omitting relevant information.
It is, therefore, an object of the present invention to provide a method and apparatus for separating a model EMG signal from a measured EMG signal that overcomes the shortcomings of conventional EMG detection/analysis techniques. The model EMG signal can be used to monitor the condition of the patient. In the case of a diaphragm EMG signal, the model diaphragm EMG signal can be utilized, for example, to synchronize the operation of a ventilator and the breathing cycles of a patient.
This object is achieved according to one embodiment of the present invention by providing a method of producing a model EMG signal from a measured EMG signal that includes a patient""s EMG signal and an ECG signal. The method includes processing the measured EMG signal to produce a logic signal that is in a first binary state in the absence of a P wave, a QRS complex, and a T wave of an ECG cycle of the measured EMG signal and in a second binary state during at least one of the P wave, the QRS complex, and the T wave of the ECG cycle. The measured EMG signal is processed to produce a first envelope signal. The model EMG signal is produced as a function of (1) the first envelope signal when the logic signal is in the first binary state and (2) the absence of the first envelope signal when the logic signal is in the second binary state.
An exemplary embodiment of the present invention contemplates that processing the measured EMG signal to produce the logic signal includes processing the measured EMG signal to produce a second envelope signal, and processing the second envelope signal to produce a fast signal. An exemplary embodiment of the present invention also processes the second envelope signal to produce a first slow signal having a slew rate that is slower than the slew rate of the fast signal. The method of the present invention processes the fast signal and the first slow signal to produce the logic signal.
An exemplary embodiment of the present invention also contemplates that processing the measured EMG signal to produce the first envelope signal includes high pass filtering the measured EMG signal to produce a high pass signal and rectifying the high pass signal to produce a rectified signal. The rectified signal is low pass filtered to produce the first envelope signal.
Producing the model EMG signal includes, in one embodiment of the present invention, providing a moving average of the first envelope signal when the logic signal is in the first binary state, and providing, when the logic signal is in the second binary state, a set value that corresponds to a value of the moving average of the first envelope signal when the logic signal changes from the first binary state to the second binary state.
A further embodiment of the method for separating a model EMG signal from a measured EMG signal according to the principles of the present invention contemplates processing the second envelope signal to produce a second slow signal having a slew rate that is slower than the slew rate of the fast signal, and processing the fast signal, the first slow signal, and the second slow signal to produce the logic signal.
The present invention also contemplates that the step of producing the model EMG signal includes continuously processing the measured EMG signal to produce a third envelope signal. When the logic signal is in the first binary state, a moving average of the first envelope signal is preferably provided. When the logic signal is in the second binary state, a moving average of the third envelope signal is provided.
It is another object of the present invention to provide an apparatus for producing a model EMG signal from a measured EMG signal, which includes a patient""s EMG signal and ECG signal, that does not suffer from the disadvantage of conventional EMG signal generating devices. This object is achieved according to the principles of the present invention by providing an apparatus that includes a logic signal processing means for processing the measured EMG signal to produce a logic signal that is in a first binary state in the absence of a P wave, a QRS complex, and a T wave of an ECG cycle of the measured EMG signal and in a second binary state during at least one of the P wave, the QRS complex, and the T wave of the ECG cycle. A first envelope processing means processes the measured EMG signal to produce a first envelope signal. An averaging means produces the model EMG signal as a function of (1) the first envelope signal when the logic signal is in the first binary state and (2) the absence of the first envelope signal when the logic signal is in the second binary state.
In an exemplary embodiment of the present invention, the logic signal processing means includes a second envelope processing means that processes the measured EMG signal to produce a second envelope signal. In addition, a fast signal processing means process the second envelope signal to produce a fast signal. A first slow signal processing means processes the second envelope signal to produce a first slow signal having a slew rate that is slower than the slew rate of the fast signal. A comparing means compares the fast signal and the first slow signal to produce the logic signal.
An exemplary embodiment of the present invention further contemplates that the first envelope processing means includes a high pass filtering means for high pass filtering the measured EMG signal to produce a first high pass signal. A rectifying means rectifies the first high pass signal to produce a rectified signal, and a low pass filtering means low pass filters the rectified signal to produce the first envelope signal.
The present invention also contemplates that second envelope processing means includes a first low pass filtering means for filtering the measured EMG signal to produce a first filtered signal. A first rectifying means rectifies the first filtered signal to produce the second envelope signal. The fast signal processing means includes a second low pass filtering means for filtering the second envelope signal to produce a second filtered signal. A first amplifying means amplifies the second filtered signal to produce a first amplified signal. A third low pass filtering means filters the first amplified signal to produce a third filtered signal and a combining means combines the second filtered signal and the third filtered signal to produce the fast signal.
An exemplary embodiment of the present invention further contemplates that the first slow signal processing means includes a second amplifying means for amplifying the second envelope signal to produce a second amplified signal. A fourth low pass filtering means low pass filters the second amplified signal to produce the first slow signal. The comparing means includes a first comparator means for comparing the fast signal and the first slow signal and which, as a function of comparison, produces the logic signal.
The apparatus further contemplates that a second slow signal processing means processes the second envelope signal to produce a second slow signal having a slew rate that is slower than the slew rate of the fast signal. The comparing means produces the logic signal as a function of the fast signal, the first slow signal and the second slow signal.
The present invention also contemplates that the second slow signal processing means includes the second amplifying means, which amplifies the second envelope signal, to produce the second amplified signal as well as a fifth low pass filtering means that low pass filters the second amplified signal to produce the second slow signal.
The comparing means include a first comparator means for comparing the fast signal and the first slow signal to produce a first comparator signal. A second comparator means compares the fast signal and the second slow signal to produce a second comparator signal. In addition, a logic gate means combines the first comparator signal and the second comparator signal to produce the logic signal.
The second slow signal processing means produces the second slow signal as a function of (1) the second envelope signal when the first comparator is in the first binary state and (2) when the first comparator is in the second binary state, a set value corresponding to the value of the second envelope signal when the first comparator changes from first binary state to the second binary state.
The present invention further contemplates that apparatus for producing a model EMG signal from a measured EMG signal includes a third envelope processing means for continuously processing the measured EMG signal to produce a third envelope signal. When the logic signal is in the first binary state, the averaging means produces a moving average of the first envelope signal, and when the logic signal is in the second binary state, the averaging means produces a moving average of the third envelope signal.
It is still another embodiment to provide an apparatus for producing a model EMG signal from a measured EMG signal that includes a patient""s EMG signal and ECG signal. The apparatus includes a first envelope processor, which processes the measured EMG signal, to produce a first envelope signal. A second envelope processor processes the measured EMG signal to produce a second envelope signal. A fast signal processor processes the second envelope signal to produce a fast signal and a first slow signal processor processes the second envelope signal to produce a first slow signal. A comparer compares the fast signal and the first slow signal to produce a logic signal which is in a first binary state in the absence of a P wave, a QRS complex and a T wave of an ECG cycle of the measured EMG signal and which is in a second binary state during at least one of the P wave, the QRS complex and the T wave of the ECG cycle. An averager produces a model EMG signal as a function of (1) the first envelope signal when the logic signal is the first binary state and (2) the absence of the first envelope signal when the logic signal is in the second binary state.
A first switch couples or isolates the measured EMG signal and the first envelope processor when the logic signal is in the first and second binary states, respectively. A second switch couples or isolates the first envelope signal and the averager when the logic signal is in the first and second binary states, respectively.
In a further embodiment, the apparatus includes a second slow signal processor that processes the second envelope signal to produce a second slow signal. The comparer also produces the logic signal as a function of the fast signal, the first slow signal and the second slow signal.
In a still further embodiment, a third envelope processor that continuously processes the measured EMG signal to produce a third envelope signal. The second switch couples the first envelope signal to the averager when the logic signal is in the first binary state and couples the third envelope signal to the averager when the logic signal is in the second binary state. The averager produces the model EMG signal as a function of the first envelope signal when the logic signal is in the first binary state and as a function of the third envelope signal when the logic signal is in the second binary state.
These and other objects, features and characteristics of the present invention, as well as the methods of operation and functions of the related elements of structure and the combination of parts and economies of manufacture, will become more apparent upon consideration of the following description and the appended claims with reference to the accompanying drawings, all of which form a part of this specification, wherein like reference numerals designate corresponding parts in the various figures. It is to be expressly understood, however, that the drawings are for the purpose of illustration and description only and are not intended as a definition of the limits of the invention.